Of course classes for Mac and Windows will do nothing on Linux, but you can still have them in the code and they will not cause compile errors. This allows cross compilations from Mac to Linux or other way around.

And in the documentation, you can click on a list of classes and than click on the Linux platform, which hides all the classes that won't work on Linux.
The Linux plugin includes the RaspberryPiCameraMBS class to get pictures on a Raspberry Pi camera, the avahi classes for finding devices on the local network, the GTK window classes, the WebKit HTMLViewer extensions and help for sudo. Enjoy!
14 03 18 - 11:08